After signing in at /login, the browser is redirected back to the authorization URL but the screen is blank.

Please make the form that appears when Javascript is enabled also work for Javascript disabled browsers. For lack of a better explanation, Lynx is still useful for OAuth flows on servers. Even if you discount that (and/or simply facepalmed), not everyone is going to have Javascript enabled at all points within the OAuth side of things (e.g. various U.S. government/military entities).